1. Field of the Invention
The present invention relates to a memory device including a semiconductor.
2. Description of the Related Art
A DRAM whose memory cell includes one transistor and one capacitor has no limit on the number of times of writing in principle, and can perform writing and reading at relatively high speed; thus, such a DRAM is used in many kinds of electronic apparatuses. A DRAM stores data by accumulating electric charge in a capacitor of each memory cell, and reads the data by releasing the electric charge (see Patent Documents 1 to 6 and Non-Patent Document 1).
DRAMs are classified into a folded bit line type and an open bit line type according to a layout of bit lines and memory cells. The area of one memory cell (cell area) of the folded bit line type DRAM is 8 F2 (F indicates a feature size) at the minimum. On the other hand, the cell area of the open bit line type DRAM can be reduced down to 6 F2 in the case of using a planar transistor, or 4 F2 in the case of using a vertical transistor. For improving integration degree, the open bit line type DRAM has been employed.
FIG. 2A is a circuit diagram of a conventional DRAM memory cell. A memory cell 201 includes a cell transistor 202 and a capacitor 203. A gate of the cell transistor 202 is connected to a word line 204a, a drain of the cell transistor 202 is connected to a bit line 205, and a source of the cell transistor 202 is connected to a first electrode (capacitor electrode) of the capacitor 203. A second electrode of the capacitor 203 is connected to a source line 206.
Memory cells with the same configuration are arranged in a matrix. A gate of a cell transistor of a memory cell adjacent to the memory cell 201 is connected to a word line 204b which is adjacent to the word line 204a. The word line 204a and the word line 204b intersect with the bit line 205.
Data is written in the memory cell 201 in the following manner. The potential of the word line 204a is controlled to turn on the cell transistor 202, and the potential of the bit line 205 is set in accordance with data to charge the capacitor 203. Then, the potential of the word line 204a is controlled to turn off the cell transistor 202. At this time, the connection point (storage node SN) of the source of the cell transistor 202 and the first electrode of the capacitor 203 has a potential corresponding to data.
Data is read from the memory cell 201 in the following manner. The potential of the word line 204a is controlled to turn on the cell transistor 202, and charge accumulated in the capacitor 203 is released to the bit line 205 in a floating state. At this time, the potential of the bit line 205 varies in accordance with the charge accumulated in the capacitor 203. Data can be read by amplification of this variation.
In order to prevent occurrence of an error during data reading, the capacitance of the capacitor 203 is required to be sufficiently larger than the gate capacitance of the cell transistor 202. During data reading, the bit line 205 is in a floating state. When the cell transistor 202 is turned on with the bit line 205 in that state, the word line 204a and the bit line 205 are capacitively coupled via the gate capacitance of the cell transistor 202, so that the potential of the bit line 205 varies.
This variation in potential is pronounced when the capacitance of the capacitor 203 is smaller than the gate capacitance of the cell transistor 202. In particular, when the capacitance of the capacitor 203 is less than or equal to ten times the gate capacitance of the cell transistor 202, a margin of data reading becomes small, leading to an error.
FIG. 2B illustrates a cross section of a DRAM including a stack capacitor. The DRAM includes an element isolation region 212 formed on a substrate 211, impurity regions 213a, 213b, and 213c formed on the substrate 211, word lines 204a and 204b, a bit line 205, capacitor electrodes 215a and 215b, and a source line 206 (also referred to as cell plate). An interlayer insulator 216 is provided between the word lines 204a and 204b and the source line 206.
The bit line 205 is connected to the impurity region 213b via a connection electrode 214b, the capacitor electrode 215a is connected to the impurity region 213a via a connection electrode 214a, and the capacitor electrode 215b is connected to the impurity region 213c via a connection electrode 214c. A stack capacitor is formed by the capacitor electrodes 215a and 215b and the source line 206.
In the case of using the circuit illustrated in FIG. 2A, it is necessary that the capacitor 203 and the bit line 205 are provided above the word lines 204a and 204b (the side of the word lines opposite to the substrate) and the bit line 205 intersects with the word lines 204a and 204b. In addition, the bit line 205 needs to be provided so as not to contact with the capacitor.
Therefore, in the open bit line type DRAM, a bit line needs to be provided not to be in contact with the connection electrodes 214a and 214c connected to the stack capacitors, as described in Patent Document 3.
In the folded bit line type DRAM, arrangement of a bit line is easy owing to a large cell area. In the open bit line type DRAM, however, space to provide a bit line between capacitors is narrow down to 1 F owing to a small cell area, so that arrangement of a bit line using a general circuit layout is difficult.
For example, when a planar transistor is used, a bit line and an element formation region need to be arranged to form an angle of approximately 29°, which is difficult to set. Even in that case, the cell area can be reduced down to only approximately 6.2 F2, and thus an ideal lower limit of 6 F2 cannot be achieved.
When a vertical transistor is used, a bit line is embedded in a substrate (i.e., an impurity region formed on a substrate, silicide formed thereover, or the like is used as a bit line). However, the thus formed bit line has a high resistance, which causes a problem of a signal delay or heat generation.